The present invention relates generally to plasma arc torch systems and power supplies. In particular, the invention relates to a circuit and method for controlling the pilot arc in a plasma arc torch.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting, welding, and spray bonding workpieces. Such torches typically operate by directing a plasma consisting of ionized gas particles toward a workpiece. An example of a conventional gas plasma arc torch is disclosed in U.S. Pat. No. 3,813,510, the entire disclosure of which is incorporated herein by reference.
In general, a pressurized gas to be ionized is supplied to the front end of the torch and flows past an electrode before exiting through an orifice in a torch tip. The electrode has a relatively negative potential and operates as a cathode. The torch tip, which is adjacent the electrode at the front end of the torch, constitutes a relatively positive potential anode. When a sufficiently high voltage is applied to the electrode, an arc is established across the gap between the electrode and the torch tip, thereby heating the gas and causing it to ionize. The ionized gas in the gap is blown out of the torch and appears as a flame extending externally from the tip. The arc so established is commonly referred to as a pilot arc. A typical pilot arc circuit may provide 5-50 amps, at 100-200 volts across the electrode to tip gap.
In order to use a plasma arc torch with a workpiece, a main or cutting arc must normally be established between the electrode and the workpiece. As the torch head or front end is brought toward the workpiece, the arc transfers between the electrode and the workpiece because the impedance of the workpiece to negative is typically lower than the impedance of the torch tip to negative. During this xe2x80x9ctransferred arcxe2x80x9d operation, the workpiece serves as the anode.
Once the arc transfer is sensed, it is generally preferred to cease current flow between the electrode and the tip. One method of terminating current flow between the electrode and the tip is to open circuit the pilot arc current path. This may be accomplished by sensing the presence of current flowing in the workpiece and open circuiting a switch between the tip and ground (positive return). Commonly owned U.S. Pat. Nos. 5,170,030, 5,530,220, and 6,369,350, the entire disclosures of which are incorporated herein by reference, describe the arc transfer process in greater detail.
After arc transfer occurs, the output current is typically increased to a higher, cutting level. The power supply preferably is current controlled so that the cutting current is maintained at or near a constant current level. If, however, the transferred arc is stretched beyond the capacity of the power supply it can extinguish. The arc may stretch, for example, when cutting a discontinuous workpiece (e.g., a metal grate), when cutting near the end of a workpiece, or when the torch is moved away from the workpiece. Once the arc has been extinguished, the torch starting process must typically be repeated. As can be appreciated, restarting the torch is relatively inefficient. Therefore, it is generally preferable to cause the pilot arc to re-attach before the transferred arc extinguishes.
U.S. Pat. No. 5,620,617 discloses an arc control circuit for a plasma arc torch. A comparator compares the output voltage of the power supply to a maximum voltage. When the output voltage exceeds the maximum voltage, the comparator sets a logic device. The logic devices generates a signal to close a switch and reconnect the nozzle to the power supply, thereby switching the arc from the workpiece to the nozzle.
U.S. Pat. No. 5,844,197 discloses an arc retract circuit for use in a plasma arc torch. The system disclosed therein involves creating a first signal representing the actual current applied by the power supply to the power circuit driving the plasma torch, creating a second signal representative of a current level below the set current level for the cutting operation, and closing a power switch in the pilot arc circuit when the first signal is essentially equal to the second signal.
The arc control/retract circuits disclosed in these patents require a comparison to a predefined reference (either voltage or current) in order to properly operate. Accordingly, such circuits are inherently limited by the reference chosen and require additional circuitry to establish the predefined reference.
For these reasons, a plasma arc torch system having an improved pilot re-attach circuit and method is desired. Such a system and method requires the creation of no additional current or voltage reference signal in order to accurately sense when to cause the pilot arc to re-attach. Further, such a system and method preferably uses an existing signal to determine when to re-attach the pilot arc. Finally, such a system and method preferably provides a reliable and repeatable method of re-attaching the pilot arc prior to the extinguishment of the transferred arc.
The invention meets the above needs and overcomes the deficiencies of the prior art by providing an improved circuit and method for accurately determining whether and when to re-attach a pilot arc when a transferred arc may no longer be sustained. Advantageously, this is accomplished by monitoring the current or voltage output of an already existing circuit associated with a standard regulated power supply. Moreover, the improvement can be accomplished without the need for creating additional reference signals for comparison purposes that are not already present in a regulated power supply.
Briefly described, a plasma arc torch system for use in connection with a workpiece embodying aspects of the invention includes a power source providing a power output current. An electrode is positioned in a circuit path with the power source. The output current flows through the electrode. A tip is adjacent the electrode. A current reference circuit provides a current reference signal that has a parameter indicative of a desired output current. A current regulator circuit provides a signal indicative of the output current. A rate of change sensor receives the output current signal. The rate of change sensor detects a rate of change in the output current signal and selectively provides a switch control signal that has a parameter indicative of the rate of change in the output current signal. A switch circuit is responsive to the switch control signal. The switch circuit selectively electrically connects the tip into the circuit path.
Another embodiment of a plasma arc torch system for use with workpiece in accordance with the invention includes a power source providing an output current. An electrode is positioned in a circuit path with the power source. A tip is adjacent the electrode. A current reference circuit provides a current output signal having a parameter indicative of the output current. A rate of change sensor is electrically connected to the output current signal and receives the output current signal and selectively provides a switch control signal in response to a rate of change in the output current signal. A switch circuit is responsive to the switch control signal. The switch circuit selectively electrically connects the tip into the circuit path with the power source and the electrode.
Another embodiment of the invention includes a pilot re-attach circuit for use in a plasma arc torch system. The torch system includes a power source that provides an output current. An electrode is positioned in a circuit path with the power source. A tip is adjacent the electrode. The pilot re-attach circuit includes a rate of change sensor that receives an output current signal and that is responsive to a time rate of change of the output current signal. The rate of change sensor provides a switch control signal having a first state when the time rate of change of the output current signal is less than a threshold. The rate of change signal has a second state when the time rate of change of the output current signal exceeds the threshold. A pilot switch is responsive to the switch control signal. The pilot switch is operable to electrically connect the tip into the circuit path with the power source and the electrode when the switch control signal is in the second state.
In still another embodiment, the invention includes a plasma arc torch system for use in connection with a workpiece. A power source means provides an output current. An electrode is positioned in a circuit path with the power source means. The electrode receives the output current. A tip is adjacent the electrode. A current sensing means provides an actual output current signal having a parameter indicative of the output current. A detector means is electrically connected to the current sensing means. The detector means receives the output current signal and selectively provides a switch control signal in response to a rate of change in the output current signal. A switching means is responsive to the switch control signal for selectively electrically connecting the tip into the circuit path with the power source means and the electrode.
Another embodiment of the invention includes a method of operating a plasma arc torch system that includes a power supply that supplies an output current, and a pilot switch that establishes a pilot arc mode of operation. An output current signal having a parameter representative of the output current is monitored for a rate of change. The pilot switch is operated when the rate of change of the output current signal exceeds a rate of change threshold such that the pilot arc mode of operation is established.
In still another embodiment, the invention includes a method of reestablishing a pilot arc in a plasma arc torch system before a transferred arc is extinguished. The plasma arc torch system includes an electrical power source providing an output current. An electrode receives the output current. A tip is adjacent the electrode. A pilot switch selectively connects the tip in a circuit path with the electrode and the power source such that when the pilot switch is closed, a pilot arc is selectively established between the electrode and the tip. The method includes monitoring an output current signal having a parameter representative of the output current provided by the electrical power source. The output current signal is monitored for a rate of change. A switch control signal is generated in response to the detected rate of change in the output current signal. The switch control signal is representative of whether the detected rate of change in the output current signal is greater than or less than a rate of change threshold. A pilot switch is operated in response to the rate of change signal such that when the rate of change is greater than the rate of change threshold, the pilot switch closes and connects the tip into the circuit path with the electrode and the power source.
Furthermore, plasma arc torch systems, pilot re-attach circuits, and methods of operating a plasma arc torch system are provided that use an output voltage signal rather than an output current signal as previously described. Accordingly, a rate of change of the output voltage signal, rather than the rate of change of the output current signal, is used to connect the tip into the circuit path when the arc is about to be extinguished.
In yet another form, a plasma arc torch system is provided that uses an output power signal rather than a current or voltage signal as previously described, wherein a power supply that does not generate power with electricity is contemplated, such as a fuel cell power supply. Similarly, a rate of change of the output power signal is used to connect the tip into a circuit path when the arc is about to be extinguished.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.